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Review of Groundwater Data



On June 27, 2001, the Agency for Toxic Substances and Disease Registry (ATSDR) received arequest from the Environmental Protection Agency (EPA) to determine the public health impact theSigmon's Septic Tank Service Facility, septage removal business, has on private wells located nearthe facility [ATSDR 2001a]. ATSDR is also reviewing the migration of chemical constituents tonearby surface water bodies (i.e., streams, creeks, ponds, etc.) and their potential impact on publichealth through recreational fishing. That evaluation will be released in a separate healthconsultation.

The Sigmon's Septic Tank Service Facility (CERCLIS No.: NCD062555792 ) is located at 1268 Eufola Road, approximately five miles southwest of Statesville, Iredell County, North Carolina [NC DENR 1998, NC DENR 2000]. The facility is active under the current name Sigmon's Environmental Services. Septic wastes are temporarily stored in four cylindrical tanks on the property, and the sludges are periodically removed and transported to a wastewater treatment plant for disposal. The business employs five workers. The work area includes the area surrounding the storage tanks (Figure 2). The owner of the business lives on the property. Drinking water for the owner's home and the business office is obtained from a private water well located on the property (Figures 1 and 2). Public access is not restricted on the south side of the property (i.e., former lagoon area/waste pile), and unauthorized persons have been reported to enter the property with recreational vehicles through breaks in the fence.

The site has been listed under several names including: Sigmon's Septic Tank Service, AAAEnterprises, and Sigmon Environmental Services (current name). Services provided by the businessowners have included the pumping and removal of septic tank wastes and heavy sludges for variouscustomers (e.g., residential, commercial, and industrial), installation and repair of septic tanks, and avariety of other waste removal services to various industries. The groundwater medium has beeninvestigated for many years and the residents are well aware of the groundwater contaminationresulting from activities at the Sigmon's Septic Tank Service Facility [NC DENR 1998, NC DENR2000]. Below is a chronological list summarizing the events that have occurred at the Sigmon'sSeptic Tank Service Facility:

Time Period Event Summary

1970 - 1978 Wastewaters from the Sigmon's Septic Tank Service Facility are originally discharged to the City of Statesville wastewater treatment plant.
1973 - 1974 Sludges from the Sigmon's Septic Tank Service Facility are land applied to area farmlands.
1978 - 1992 Sigmon's Septic Tank Service begins disposing of septic wastes within ten on-site lagoons. Within time period, the North Carolina Department of Natural Resources and Community Development--Division of Environmental Management (DEM) collected groundwater samples from on-site monitoring wells and from nearby private water wells. Analysis revealed elevated levels of metallic and organic chemicals.
1992 DEM and the North Carolina Hazardous Waste Section conduct a site investigation and sampling trip to determine if the wastes in the on-site lagoons are hazardous. Since the chemical constituents of the on-site lagoons did not meet the definition of a hazardous waste, the North Carolina Hazardous Waste Section decides the site does not fall under its jurisdiction and refers it to the North Carolina Solid Waste Section for further evaluation.
1992 - 1995 On-site lagoons are non-operational.
1995 DEM requires the on-site lagoons to be closed. Lagoon sludges are excavated and piled in lagoon area.
Dec. 1995 DEM refers site to the North Carolina Superfund Section regarding removal options of the piled sludge in lagoon area.
Dec. 1996 North Carolina Superfund Section adds the site to the CERCLIS database as one for further investigation.
Jan. 1997 North Carolina Superfund Section refers the site to the EPA Emergency Response and Removal Branch for removal evaluation.
Apr. 1997 EPA concludes that the site does not meet their criteria for removal eligibility.
Aug. 1997 North Carolina Superfund Section initiates its investigation of the Sigmon site by conducting sampling in a combined Preliminary Assessment/Site Inspection (PA/SI). Collected samples from the waste pile, open pits, former lagoon area, storage tank area, surface water pathways, on-site/off-site monitoring wells, and nearby private wells.
Dec. 1999 North Carolina Superfund Section conducts additional sampling at the site through an Expanded Site Inspection (ESI). Again, collected samples from the waste pile, former lagoon area, surface water pathways, an on-site monitoring well, and nearby private wells.

The groundwater pathway appears to be of great concern to nearby private well users. Within one-quarter mile of the Sigmon property, 14 people have some degree of chemical contamination in theirwell water. Several residents have been advised by the North Carolina Occupational andEnvironmental Epidemiology Section that water from their private wells should not be used for drinking purposes [NC DENR 1998].

Large amounts of waste still remain on site, which the North Carolina Superfund Section believes isthe source of the contamination. They have recommended to EPA that a non-critical removal beconducted to address the remaining source areas at the site. This would result in further mitigatingany potential chemical releases to the soil, groundwater, and surface water pathways [NC DENR 2000].


ATSDR reviewed analytical results of collected groundwater samples to determine if chemicalreleases to the area's groundwater may be impacting the public health of nearby private well users [NC DENR 1998, NC DENR 2000].

Environmental Monitoring Data

In September 1987, four on-site monitoring wells (MW1, MW2, MW3, and MW4) were installedaround the lagoon area at depths of 34 to 39 feet. Figures 1 and 2 give an aerial perspectiveillustrating the location of the on-site monitoring wells (refer to label MW1A in Figures 1 and 2) inrespect to the source areas, nearby private wells, and other sampling locations (i.e., an off-sitemonitoring well). Figure 3 gives a more detailed areal perspective of the on-site monitoring wells in proximity to the closed lagoons.

Between 1987 and 1999, 12 samples were collected from the on-site monitoring wells andsubsequently analyzed for chlorides, nitrates, sulfates, metals, volatile organic compounds (VOCs),and semi-volatile organic compounds (SVOCs). Table 1 (see Appendix B) summarizes theseanalytical results and compares them to drinking water comparison values (CVs, see Appendix A),assuming both short-term (i.e., acute or intermediate) and long-term (i.e., chronic) exposures.

Data from the monitoring wells were not used to determine exposure levels since the most likelypoints of exposures are private wells. Monitoring well data was reviewed to determine which of thechemicals from the site could potentially impact private wells. Of the 40 chemicals detected in on-site monitoring wells, only 12 (30 percent) were detected at levels above drinking water CVs. Six ofthe twelve chemicals (i.e., aluminum, arsenic, barium, sodium, benzene, and vinyl chloride) werenot detected in private wells at levels above drinking water CVs. Therefore, these six chemicals willnot be evaluated any further in this health consultation.

Off-site Monitoring Well

During the August 1997 PA/SI, a groundwater sample was collected from an off-site monitoringwell (see Figure 1), located approximately 100 feet southeast of the closed lagoons. Chemicalanalysis of the collected groundwater sample showed no chemical detects above their respectivedetection limits except for acetone (31 parts per billion). However, that reading was thought toreflect lab contamination and was below drinking water CVs (refer to Table 2 in Appendix B). Aslong as several source areas (i.e., closed lagoons, waste piles, and open pits) remain at the site,routine collection and analysis of groundwater samples is advisable.

Private Wells

Chemical analyses of collected samples from the on-site monitoring wells did indicate that the closedlagoons and the waste piles have been, and may still be, sources of contaminants that leach into thearea's groundwater. These contaminants may subsequently migrate into nearby private wells. Suchmigration may account for contamination found in at least two private wells. Samples collectedfrom the two private wells showed chemical detects of mercury, nitrates, and VOCs, that were alsodetected in the on-site monitoring wells. For these two private wells, chemical constituents weregenerally detected at higher concentrations than in other nearby private wells. Some of the chemicallevels detected in the two private wells also exceeded drinking water CVs. Because source areasstill remain at the site and because it is likely that they may have impacted at least two private wells,consideration should be given to removing these source areas from the site to prevent or mitigate anypotential migration of chemicals into nearby private wells.

Between 1991 and 1999, groundwater samples were collected from 11 nearby privates wells, withone well being abandoned due to a hazardous waste spill unrelated to the site. Approximately 36samples were collected and subsequently analyzed for nitrates, sulfates, metals, VOCs, and SVOCs. Tables 3 through 13 (see Appendix B) summarize the analytical results and compares them todrinking water CVs. The analytical results for each well are summarized in the followingparagraphs.

  • Private Well PW2
  • Private Well PW2 (see Figures 1 and 2) is topographically down gradient of theSigmon source areas (i.e., 400 feet southwest of the lagoon area) and serves at leastone person. The drilled depth is unknown. Twenty-one of the 23 detected chemicalswere also detected on-site (most notably mercury, nitrates, and VOCs). Thus,Private Well PW2 is probably being impacted by the source areas located at the site.

    Of the 23 chemicals detected in samples collected from the Private Well PW2,maximum detected concentrations for six chemicals (i.e., nitrates, iron, manganese,mercury, 1,4-dichlorobenzene, and 1,2-dichloroethane) exceeded drinking waterCVs (see Table 3).

  • Private Wells PW1A, PW1R, PW1S
  • In an area southeast of the site, a relatively large parcel of property contains a private residence, a business, and four rental houses. Three private wells are also located onthe property approximately 1,000 feet east of the lagoon area. One of the privatewells (PW1R) provides water for the residence and business. The other two privatewells furnish water for the four rental houses, and water samples were collected fromone of those wells (PW1S). The private wells furnishing water for the rental housesare approximately 60 feet deep. The owner of the property also installed twomonitoring wells, located approximately 100 to 150 feet southeast of the lagoonarea. As previously mentioned, the monitoring well approximately 100 feetsoutheast of the site was sampled during the August 1997 PA/SI. Another well(PW1A) on the property, approximately 200 feet east of the lagoon area, wasabandoned in 1994 because a tenant of a house closest to the well reportedlycontaminated it with gasoline.

    Of the 12 chemicals detected in samples collected from Private Wells PW1A,PW1R, and PW1S (see Tables 4 through 6), the detected levels of only twochemicals (i.e., iron and manganese) measured during a 1992 sampling event fromthe abandoned well (PW1A) exceeded drinking water CVs (see Table 4). Eventhough the levels of iron and manganese exceeded drinking water CVs, it is of nopublic health concern now because the well in question is abandoned and water fromthat well is not being used for either drinking or non-drinking purposes that couldlead to exposure [NC DENR 1998, NC DENR 2000].

  • Private Well PW33
  • Private Well PW33 is located approximately 800 feet northwest of the lagoon areaand serves at least two people (see Figures 1 and 2). The bored well isapproximately 56 to 62 feet deep. Of the ten chemicals detected in samples collectedfrom the well, only the maximum detected concentration of lead exceeded itsrespective drinking water CV (see Table 7).

  • Private Well PW5
  • Of the five chemicals detected in a sample collected from Private Well PW5 (east ofthe site; see Figure 2), drinking water CVs were available for only four. However,these four chemicals were not selected for further public health evaluation becausenone of the detected levels exceeded their respective drinking water CVs (see Table 8).

  • Private Well PW3
  • Private Well PW3 (see Figures 1 and 2) is topographically down gradient of theSigmon source areas (i.e., 450 feet southwest of the lagoon area) and serves at leastone person. The bored well is approximately 27 feet deep. Sixteen of the 19chemicals detected from samples were also detected on-site (most notably mercury,nitrates, and VOCs). Thus, Private Well PW3 is probably another well that is beingimpacted by the source areas located at the site.

    Of the 19 chemicals detected in samples collected from Private Well PW3,maximum detected concentrations for seven chemicals (i.e., nitrates, manganese,mercury, bromodichloromethane, chloroform, dibromochloromethane,1,4-dichlorobenzene) exceeded drinking water CVs (see Table 9).

  • Private Well PW32
  • A private well is located at the facility that is used for drinking purposes. The well is located approximately 1,200 feet north of the lagoon area and serves at least twopeople (see Figure 1). Of the two chemicals detected in the well, none of the detectedlevels exceeded any available drinking water CVs (see Table 10).

  • Private Well PW4
  • Private Well PW4 is located approximately 1,100 feet northeast of the lagoon areaand serves at least one person (see Figure 1). The bored well is approximately 50feet deep. The sample collected during the 1997 PA/SI was intended to be used as abackground sample.

    Of the nine chemicals detected in samples collected from the well, maximumdetected concentrations for two chemicals (i.e., lead and 1,4-dichlorobenzene)exceeded drinking water CVs (see Table 11).

    The maximum detectable level of 1,4-dichlorobenzene was measured in the wellduring the 1997 PA/SI. The validity of this measurement is in doubt because thelevel via the extractable method was used, while the more reliable purgeable methoddid not record any VOC levels in the well for this chemical or any other volatilechemicals at a detection limit of 5 ppb.

  • Private Well PW34
  • Private Well PW34 is located approximately 600 feet east of the lagoon area andserves at least three people (see Figure 1). The well is 363 feet deep. Of the twochemicals detected in a sample collected from the well, none of the detected levelsexceeded any available drinking water CVs (see Table 12).

  • Private Well PW6
  • Private Well PW6 is located north of the site and was used as the background wellfor the 1999 ESI (see Figure 2). Of the six chemicals detected in a sample collectedfrom the well, only five had available drinking water CVs. These five chemicalswere not selected for further public health evaluation because none of the detectedlevels exceeded their respective drinking water CVs (see Table 13).

Of the samples collected from 11 nearby private wells surrounding the site, ten chemicals (i.e.,Nitrates; Lead; Mercury; Iron; Manganese; 1,4-Dichlorobenzene; 1,2-Dichloroethane;Bromodichloromethane; Chloroform; and Dibromochloromethane) were selected for further publichealth evaluation because their maximum detect levels exceeded drinking water CVs (see Table 14).

Three additional substances (i.e., calcium; magnesium; potassium) were also selectively screened forfurther public health evaluation because there are no available drinking water CVs for theseelements (see Table 14). All three of these substances are essential elements required for normalhuman growth and maintenance of health. Calcium is required for the development of strong bones,magnesium is an essential cofactor of many enzymes, and potassium is important in the transmissionof nerve impulses.

The estimated maximum daily intake rates of these elements for an adult and a child who consumewater containing the maximum chemical levels found in the private wells sampled near theSigmon's Septic Tank Service Facility, are listed below. These estimated intake rates are comparedto the elements' Recommended Daily Allowance (RDA) Ranges that include children and adults ofvarious age groups. The estimated intake rates are too low to even meet the minimal physiologicalrequirements as set by the RDAs. Therefore, these three elements (i.e., calcium, magnesium, andpotassium) will not be evaluated any further in this health consultation.

RDA Comparison
Element Estimated Daily Intake Rate (mg/day) RDA Range (mg/day)
Adult 1 Child 2




800 - 1,200




150 - 350




1,600 - 3,500

1 Assuming the average adult weighs 70 kg (i.e., 150 lbs.) and consumes 2 liters of water per day.

2 Assuming the average child weighs 10 kg (i.e., 20 lbs.) and consumes 1 liter of water per day.

Exposure Pathways

Chronic or long- term exposure to chemicals in the groundwater can occur via ingestion, inhalation(i.e., VOCs), and dermal contact, when groundwater is used for drinking, showering, bathing, andother household purposes. Studies indicate that significant exposures to VOCs can occur duringthese activities as the chemicals volatilize, and are then subsequently inhaled and/or absorbedthrough the skin. These exposures to VOCs may equal or exceed those from ingestion, usually, byno more than a factor of 2 [Jo et al 1988, Kerger & Paustenbach 2000, Kezic et al 1997, Mattie et al1994, EPA 1999].

For the purposes of this health consultation, the primary route of human exposure is considered to beingestion. Inhalation exposure was determined to have no public health implications because of thefollowing: (1) VOCs were estimated to be released into the air at very low levels (i.e., 0.0024 partsper million for 1,2-dichlorobenzene, 0.0022 parts per million for 1,4-dichlorobenzene, 0.0001 partsper million for 1,1-dichloroethane, etc.) during showering and other uses [Andelman 1990], (2) theestimated VOC levels in air (i.e., a conservative approximation assuming the worst case scenario)were well below CVs for inhalation exposures that considered both short-term and long-termexposures, (3) a dermal study indicated that 2% to 5% of organic chemicals (i.e., non-polarcompounds) in an aqueous matrix are absorbed through the skin during a 30-minute period[Webster et al 1987]; therefore, the absorption of organic chemicals through dermal exposure fromshowering under such conditions (i.e., low-level water concentrations) is also considered to benegligible, and (4) the detected metals in water will neither volatilize into the air nor be absorbedthrough the skin because the detected metals are dissolved in solution and not so readily released.

Public Health Implications

Based on ATSDR's review of the groundwater sampling and analysis data, the following chemicalswere selected for further public health evaluation: nitrates; lead; mercury; iron; manganese;1,4-dichlorobenzene; 1,2-dichloroethane; bromodichloromethane; chloroform; anddibromochloromethane. With the exception of nitrates, these chemicals were classified as metals(i.e., lead, mercury, iron, manganese) or VOCs (i.e.; 1,4-dichlorobenzene; 1,2-dichloroethane;bromodichloromethane; chloroform; dibromochloromethane).


The toxicity of nitrates is due to its conversion to nitrites by bacteria in the gastrointestinal tract (i.e.,intestines). Infants are especially susceptible to methemoglobinemia because the higher pH of theirgastric juice is more compatible with the growth of nitrate-reducing bacteria in the gut. Olderchildren, with their more acidic gastric juices, are much less susceptible [Craun et. al. 1981]. Nitrite oxidizes the Fe(+2) of iron in hemoglobin to the Fe(+3) state. The resulting compound(methemoglobin) does not bind oxygen, so that the blood cannot transport as much oxygen from thelungs to the tissues. Infants are the particularly sensitive to nitrate/nitrite toxicity. The characteristicblueness (cyanosis) of lips and mucous membranes, which generally precedes the adverse symptomsof methemoglobinemia, can be produced by methemoglobin levels as low as 10%. Methemoglobinlevels under 30% produce minimal symptoms (fatigue, lightheadedness, headache) in healthychildren and adults, while levels between 30% and 50% cause moderate depression of thecardiovascular and central nervous systems (weakness, headache, rapid breathing and heartbeat,mild shortness of breath). Levels between 50% and 70% cause severe symptoms (stupor, slow andabnormal heartbeat, respiratory depression, convulsions), and levels above 70% are usually fatal[Ellenhorn, et. al., 1988]. Any levels of methemoglobin that might be associated with the maximumdetected nitrate levels in water from private wells at this site are likely to be less than 2%. (Seediscussion below.)

EPA has developed a chronic oral reference dose for the ingestion of nitrates based on the earlyclinical signs of methemoglobinemia (cyanosis) in infants ingesting water containing varyingconcentrations of nitrate-nitrogen. That RfD is set equivalent to the observed NOAEL (i.e., NoObserved Adverse Effect Level) of 1,600 g nitrate-nitrogen/kg/day which is the dose that would bereceived by a 0-3 month old infant weighing approximately 8.8 pounds (4 kg) and drinking 0.64liters/day of water (as formula) containing 10,000 ug/L nitrate-nitrogen.

One primary source of organic nitrates is human sewage, which is the type of business conducted atthe site (i.e., removal and handling of septic wastes). Due to their high solubility and weak retentionby soil, nitrates and nitrites are very mobile in soil and have a high potential to migrate togroundwater. Most nitrogenous materials in natural waters tend to be converted to nitrate, so allsources of combined nitrogen, particularly organic nitrogen and ammonia, should be considered aspotential nitrate sources. Because it does not volatilize, nitrate/nitrite is likely to remain in wateruntil consumed by plants or other organisms. Ammonium nitrate will be taken up by bacteria.Nitrate is more persistent in water than the ammonium ion. Nitrate degradation is fastest inanaerobic conditions.

Nitrate was detected at levels above drinking water CVs in two private wells, Private Wells PW2and PW3, near the site. The estimated daily dose of nitrate from water containing 23,350 ppb(maximum nitrate detection in Private Well PW2) would be 667 g/kg/day for a 70 kg (i.e., 150pounds) adult ingesting two liters of water per day; 2,335 g/kg/day for a 10 kg (i.e., 20 pounds)child ingesting one liter of water per day; and 3,736 g/kg/day for a 4 kg (i.e., 8 pounds) infantingesting 0.64 liters of water (as formula) per day. Although the estimated daily dose for a child isslightly higher than the RfD, non-cancerous health effects are not expected in adults or childrenolder than six months at these dose levels. Although it is not recommended that infants 1-3 monthsof age be chronically exposed to levels of nitrates that exceed EPA's RfD (in this case, by a factor of2.3), adverse effects would not be likely to occur in them, either. In one study, oral doses of nitrateranging from 100 g/kg/day to 15,500 g/kg/day in 111 infants less than six months old wasassociated with methemoglobin levels as high as 5.3% (mean 1.6%), but none of the children hadthe typical symptoms of methemoglobinemia [Winton, et. al., 1971]. In another study, meanmethemoglobin levels were only 1.3% in infants aged 1-3 months who received water containing11,000-23,000 µg nitrate-nitrogen/L [Simon et al., 1964]. No clinical signs of methemoglobinemiawere detected in any of these infants, either. Low levels of methemoglobin (0.5 to 2.0%) occurnormally and, due to the large excess capacity of blood to carry oxygen, levels of methemoglobin upto 10% are seldom associated with any clinically significant signs such as cyanosis (IRIS 2001). Most cases of infant methemoglobinemia are associated with exposure to nitrate in drinking waterused to prepare infants' formula at levels >20,000 ppb of nitrate-nitrogen. However, cases havebeen reported at levels of 11,000-20,000 ppb nitrate- nitrogen, especially when associated withconcomitant exposure to bacteriologically contaminated water or excess intake of nitrate from othersources. Therefore, if other sources of drinking water are available, well water from Private WellPW2, and possibly Private Well PW3, should not be used for making infant formula.

On July 11, 2001, an ATSDR regional representative learned that a six-month-old baby lived in ahome located on the same street as Private Wells PW2 and PW3, probably impacted by the sourceareas located on site [ATSDR 2001c]. During the 1997 PA/SI, the well for this home was not testedbecause there was no response to NC DENR messages left at the door. However, the ATSDRregional representative learned from one of the occupants that they used bottle water for bothcooking and drinking. The same ATSDR regional representative later learned that the watersupplied to the home is from Private Well PW1R [ATSDR 2001e]. Past sampling (i.e., 1997PA/SI) has indicated that no chemicals from the site have impacted Private Well PW1R. Initially,EPA planned to sample the well supplying water to the home in question during November 2001;however, upon learning that the well water originated from Private Well PW1R, they concludedthere was no immediate need to resample the well because it was not contaminated [ATSDR 2001d,ATSDR 2001e]. (Note, detected nitrate levels in Private Well PW1R were no higher than 1,400ppb as of August 1997.) Even though the water from Private Well PW1R is probably safe for aninfant to consume, ATSDR recommends that EPA periodically collect and analyze groundwatersamples, especially for nitrates, for households with infants and small children until the sources areasare removed from the site.


Lead, mercury, and two nutrient metals, iron and manganese, were detected in several private wellsnear the site. The public health implications of these metals are discussed below.


The EPA Office of Drinking Water has established 15 ppb as an action level for lead in drinkingwater [EPA 1991]. For children and adults drinking water with lead levels at 1-14 ppb, no action isnecessary; however, children and pregnant females, should stop drinking the water if it contains leadlevels at 15 ppb or greater. Furthermore, one may want to consider not using the water for cooking,especially if children and pregnant females reside in the household. Adults drinking water with leadlevels of 15 to 50 ppb should try to reduce their consumption, and water containing lead levels at 50ppb or greater should not be used for either drinking or cooking.

Lead was detected in five private wells near the Sigmon's Septic Tank Service Facility; however,only two wells (Private Wells PW33 and PW4) contained lead levels (17 ppb and 28 ppbrespectively) that exceeded EPA's Lead Action Level of 15 ppb. EPA's Lead Action Level of 15ppb was exceeded only once in each well between 1994 and 1999. Intermittent exposures of thistype (i.e., limited and infrequent excursions above the action level of 15 ppb) over an extendedperiod of time (e.g., more than a year) are not likely to be associated with any adverse health effects. As estimated by EPA's Integrated Exposure Uptake Biokinetic Model for Lead in Children(IEUBK), a blood lead level increase of 2 g/dL is expected in children drinking water containingthe maximum lead level (28 ppb) found in the Private Well PW4 [EPA 2001]. Inasmuch as averageblood lead levels in the U.S. (i.e., levels not associated with toxicity) fell by five times that amount(from 12.8 to 2.8 g/dL) in the 1980s (i.e., between NHANES II and III), an increase of 2 g/dL isnot likely to be of any toxicological significance. However, since it is the total blood lead level, andnot some increment, that is associated with health risks, concerned residents should ask their localphysicians to determine their blood lead levels.

The health effects of lead are not immediately apparent. Once in the blood, lead is distributed to thesoft tissue (kidneys, bone marrow, liver, and brain) and mineralizing tissue (bones and teeth). Bonesand teeth contain about 95% of the total body burden of lead [ATSDR 1999b]. It is the level of leadin the blood that is related to the risk of adverse health effects, and the small amounts of lead that arereleased from the bones over time contribute to those blood levels. Thus, cumulative, low-levelexposures, as well as higher acute exposures, can be of potential health concern, especially inpregnant women. However, CDC's current limit of 10 ug/dL is designed to be protective of thepublic's health and does not constitute an established level of toxicity. Exposure to very high levelsof lead can cause anemia and encephalopathy (80-100 g/dL), kidney damage in adults (40 - 100g/dL) and children (35 - 50 g/dL), and increased blood pressure in adult males (30 g/dL)[Goyer 1996, Table 23-5, pg 705]. Acute effects of exposure to high lead levels are nausea,vomiting, and headache. High levels of blood lead (40 g/dL) may affect sperm or damage otherparts of the male reproductive system making it difficult for a couple to have children [ATSDR1999b].

Certain subgroups of the population may be more susceptible to the harmful effects of leadexposure: preschool age children (< 6 years old), pregnant women and their fetuses, and the elderly.Other susceptible people may include those with genetic diseases affecting heme synthesis (acomponent of the blood), nutritional deficiencies (especially iron and calcium), and neurological orkidney dysfunctions. Smoking cigarettes and drinking alcohol also may increase the risk ofnoncancerous health effects to lead exposure [ATSDR 1999b].

EPA has concluded that the human data are inadequate to determine if lead exposure could causecancerous health effects in people. However, based on sufficient evidence in animals and inadequateevidence in humans, EPA has classified lead as a probable (B2) human carcinogen.

A simple medical test is available for screening blood lead levels. People who are concerned abouttheir exposure to lead should see their doctor for more information. In addition, there are a numberof short-term remedies that you can take to reduce the lead concentrations in your drinking water and, thus, your exposure to lead.

If the source of lead is the plumbing, let the water run from the tap for from 30 seconds totwo minutes before using it for drinking and cooking. The longer water stays in water pipes,the more lead may have dissolved out of the lead pipes. Water that has been in the pipes formore than four hours should be flushed for three to five minutes, for example, first thing inthe morning and when you arrive home in the evening. A good indication of when to stopflushing the cold water tap is when the water becomes noticeably colder. Use cold water for cooking or making infant formula because water from the hot water tap tends to dissolve lead more quickly, which will cause lead concentrations to be higher in hot water.

If the source of lead is the groundwater and your tap water contains lead in excess of 15 ppbeven after flushing, then you may want to consider using bottled water instead of tap waterfor drinking or cooking purposes. Alternatively, you may choose to use a water purificationsystem. Purification systems range in size and cost from the water pitcher filtration systems to purification systems for the entire household.


Mercury was selected for further analysis because the average (2.8 ppb) and maximum (7 ppb)levels detected in drinking water wells at the Sigmon site exceeded the MCL of 2 ppb (Table 14),but only marginally so, relative to built-in margins of safety. (The RfD for mercuric chloridecontains an uncertainty factor of 1000.) ATSDR currently has no comparison values specific forinorganic mercury. However, concentrations of mercury in groundwater at the Sigmon site did notexceed ATSDR's CVs for either chronic or intermediate duration exposure to mercury chloride viathe oral route in adults. More importantly, the dose (0.2 g/kg/day) that would result if a 70-kgadult (i.e., 150 pound adult) consumed 2 liters of water per day containing the maximum level ofmercury (7 ppb) detected near the Sigmon's Septic Tank Facility is at least 1000 times lower thanthe lowest known LOAELs for inorganic mercury (the predominant form in water) in humans oranimals exposed via the oral route [ATSDR 1997, Table 2-2]. This maximum estimated dose isalso below all known LOAELs for organic mercury in humans or animals exposed via the oralroute. Therefore, none of the levels of mercury detected in groundwater at the Sigmon site would beexpected to produce adverse health effects of any kind in exposed residents.

Nutrient Metals: Iron & Manganese

The Environmental Protection Agency (EPA) has established non-enforceable Secondary DrinkingWater Guidelines (SDWGs) to maintain the aesthetic quality of water, including its taste and odor. EPA's SDWGs for iron and manganese are 300 and 50 ppb, respectively. At concentrations abovethe SDWGs, iron and manganese may cause undesirable tastes, deposit on foods during cooking,and leave reddish-brown (iron) or brownish-black (manganese) stains on plumbing fixtures andlaundry. The concentrations of iron and manganese in some of the residential wells surrounding theSigmon's Septic Tank Facility were above these non-health-based standards.

The water samples collected from nearby private wells surrounding the site contained from 14 to5,500 ppb iron. Water containing about 300 ppb of iron or more may not taste very good, but evenat 5,500 ppb iron, no adverse health effects would be expected. Iron is among the most abundantelements present naturally in the Earth's crust, and there is very little risk of toxicity from iron innatural foods and water. This is because (1) iron is an essential element in the human diet and (2)the body has a number of mechanisms specifically designed to maintain a relatively constant bloodlevel in the face of wide variations in dietary intake [Goyer, 1996, pp 715-16]. The recommendeddaily allowance of iron for adult males and females of reproductive age is 10 and 18 mg,respectively. (As noted previously, the secondary MCL of 0.3 mg/L is based on taste andappearance, and not on any potential for adverse health effects.) The long-term toxic levels ofdietary iron seen in most monogastric animals (i.e., those with a single stomach) are generally 340 to1,700 times greater than the nutritional requirement for humans [NRC 1980, pp 309-12,and TableV-12 on page 320]. By comparison, for women of reproductive age whose iron requirements aremet entirely by food alone, and who additionally consume 2L/day of well water containing 5,500ppb iron, the total daily dietary intake of iron would be little more than twice the nutritionalrequirement. Thus, while iron toxicity is a possibility under certain circumstances, drinking water isseldom the source of toxic exposures.

Manganese (Mn) is another nutrient metal with very limited toxic potential via the oral route. (Adverse effects in humans resulting from manganese exposure are associated primarily withinhalation exposure in occupational settings such as mining.) The levels of manganese in privatewells in the vicinity of the site ranged from 4.2 to 830 ppb. Assuming consumption of 2L/day foradults and 1L/day for children, the highest concentration of manganese detected in wells around thesite (830 ppb) would correspond to a daily intake of only 1.66 mg Mn/day for adults and half thatfor a small child. These amounts are of little consequence when compared to safe normal dietaryexposures. The World Health Organization (WHO) estimates that the average daily intake of Mnranges from 2 to 8.8 mg Mn and that 8-9 mg/day is perfectly safe. A normal diet, especially avegetarian diet, may contain well over 10 mg Mn/day or 0.14 mg Mn/kg/day for a 70 kg adult[NRC 1980]. The maximum concentration detected does exceed the secondary MCL of 50 ppb, butthis MCL, like the one for iron, is based solely on aesthetic considerations. (At concentrations over2,000 ppb, Mn precipitates upon oxidation and causes undesirable tastes, deposits on foods duringcooking, and leaves black stains on plumbing fixtures and laundry.) Therefore, the Mn levelsdetected in private wells around the site are expected to have no public health implications.


Based on the private well sampling data, the concentrations for none of the detected VOCs exceededEPA's MCLs or any CVs for non-cancer effects. The maximum concentrations of five VOCs (bromodichloromethane, chloroform, dibromochloromethane, 1,4-dichlorobenzene, and 1,2-dichloroethane) did exceed their respective cancer-based CVs for drinking water. These CVs werebased on studies in which laboratory animals were force-fed very high, single, daily doses of thesubstance in oil over most of the animals' lifetimes, an experimental practice that maximizes theinstantaneous assault on the animals' defense systems and increases the likelihood that toxic effectswill be produced. However, humans are exposed to chloroform and other chlorination by-productsin drinking water (not in gavage oil) and substances like chloroform and dibromochloromethane donot cause cancer in laboratory animals when they are administered in drinking water. This isprobably because (a) the substances are less soluble in water than in oil and (b) the total daily dosein humans is spread out over the entire day so that each individual dose is much smaller (and,therefore, more easily detoxified) than the single daily dose administered to laboratory animals. Therefore, ATSDR considers that neither cancer nor non-cancer effects would be expected to occuras a result of site-specific exposures to VOCs in drinking water at the Sigmon site, even with alifetime of exposure.

Assessment Limitations

The following issues should be noted regarding the groundwater contamination found near the Sigmon's Septic Tank Service Facility:

  • Unknown aquifer conditions - ATSDR was not provided enough information to identifythe site groundwater flow direction and groundwater level fluctuations with seasonalvariations. Knowing the site groundwater flow direction would give better insight to whichprivate wells are actually being impacted by the source areas located at the site. Furthermore, the groundwater plume has not been properly delineated to determine wherethe minimum and maximum concentration levels are located within the area's groundwater.

  • Unknown lead sources - Lead was detected in five private wells near the Sigmon's SepticTank Service Facility; however, only two wells (Private Wells PW33 and PW4) containedlead levels (17 ppb and 28 ppb respectively) above EPA's Lead Action Level of 15 ppb. Both of these wells are topographically up gradient (i.e., "uphill") of the source areas locatedat the site. Also, at the time these lead levels were detected (i.e., 1997 PA/SI), they werehigher than the lead level (12 ppb) detected in on-site monitoring well MW 1A. Therefore,even though lead is a site contaminant, it is probable that other attributable sources, such asplumbing (lead piping, lead-based solder, and water faucets containing lead), wereresponsible for the high measurements. These varied results could also indicate thepossibility of discrepancies in sampling location (i.e., indoor or outdoor tap samplecollection) or faucet run-time.

  • Unknown well construction quality - Because the Sigmon's Septic Tank Service Facility islocated in a rural area, it was difficult to determine when some of these private wells wereconstructed. Because the construction details were not provided, it was also hard to verifythe wells' integrity. (Note that contaminants may enter poorly constructed wells more easily than wells of good quality.)

  • Differences in well sampling plans - Of the 11 private wells sampled between 1991through 1999, there was no consistency in the number of samples collected from each well,ranging from one sample per well to ten samples per well. In addition, each sampling event(e.g., 1997 PA/SI, 1999 ESI, etc.) did not sample all of the same wells, sometimes evenchanging the well used to measure background levels. For both the 1997 PA/SI and 1999ESI, the chemical analysis methods seem to be the same, analyzing for metals, SVOCs, andVOCs; however, it is unknown which methods were used for analysis of the other samplescollected before the 1997 PA/SI. Inconsistencies such as these can be alleviated if a uniformsampling plan is adhered to, one in which the same set of wells and the same methods ofchemical analysis are employed for each sampling event.

  • Unknown origin of chlorination by-products - Three VOCs (i.e., the trihalomethanesbromodichloromethane, chlorodibromomethane, and chloroform) detected in the PrivateWell PW3 (note, only chloroform was detected in Private Well PW4) are common by-products of the chlorination of drinking water. None of these chemicals were detected in theon-site monitoring wells. While chlorination of the well water would be a plausible sourceof the observed trihalomethanes, it is unknown whether an alternative method (or no methodat all) was used to purify the well water. It is also possible that the trihalomethanes leachedfrom septic tank leaching fields. Chlorination is an effective means of treating drinkingwater for microbial agents (e.g., coliform, cryptosporidium, giardia lambia, etc.); however,guidelines are set to ensure that the chlorination of non-municipal water sources is donecorrectly. Therefore, it may be wise to contact your county health department to determine if the proper guidelines for chlorinating such water sources are being followed.

  • Unknown presence of microbial agents - Drinking water quality can also be affected bythe presence of microbial agents. Unpleasant taste, odor, and color of water are not onlycaused by elevated levels of metals such as iron and manganese, but by some types ofbacteria as well. Due to the type of business conducted at the Sigmon's Septic Tank ServiceFacility, there may be microbial agents migrating from the source areas located at the site. ATSDR contacted the Iredell County Health Department and was informed by arepresentative that none of the private wells near the Sigmon's Septic Tank Facility havebeen sampled/analyzed for fecal and total coliform counts. The representative stated that theIredell County Health Department would provide such an analysis upon the request of anyconcerned well owner who feels their well has been contaminated by microbial agents.


As part of ATSDR's Child Health Initiative, ATSDR considers children in the evaluation for allenvironmental exposures and uses health guidelines that are protective for children. Whenevaluating any potential health effects via ingestion, children are considered a special populationbecause their lower body weight causes an increased body burden (i.e., higher exposure doses),which can make them more susceptible to adverse health effects via chemical exposure. Averagebody weight differences, as well as average differences in child-specific intake rates for variousenvironmental media, are taken into account by ATSDR's child Environmental Media EvaluationGuides (EMEGs).

The maximum levels of nitrate detected (23,350 ppb) could pose an increased risk of highermethemoglobin levels to very young infants (less than six months of age) drinking formula preparedwith private well water. However, the information available to ATSDR suggests that no infantslived in the homes serviced by the wells containing high levels of nitrate. Nevertheless, as a matterof prudent public health policy, ATSDR recommends that households with infants and smallchildren, have their well water tested periodically to assure that the concentrations of nitrates and lead are within safe drinking water standards.


  1. At the concentrations detected between 1991 and 1999, the chemicals identified in thefollowing private wells surrounding the Sigmon's Septic Tank Service Facility, pose noapparent public health hazard to area residents using these wells: Private Wells PW1A,PW1R, PW1S, PW5, PW6, PW32, and PW34.

  2. Private Wells PW2 and PW3 showed nitrate levels greater than 10,000 ppb. This couldpose an increased risk of higher methemoglobin levels in very young infants (0 to 6 months) drinking formula prepared with water from these wells.

  3. Private Wells PW33 and PW4 contained lead levels that exceeded EPA's Lead ActionLevel of 15 ppb at least once in each well between 1994 and 1999. Intermittently elevatedexposures of this type (i.e., limited and infrequent excursions above the lead action level) over an extended period of time are not likely to produce adverse health effects.

  4. ATSDR identified several limitations in the site investigations regarding groundwatercontamination found near the Sigmon's Septic Tank Service Facility: 1) Unknown AquiferConditions, 2) Unknown Lead Sources, 3) Unknown Well Construction Quality, 4)Differences in Well Sampling Plans, 5) Unknown Origin of Chlorination By-products, and6) Unknown Presence of Microbial Agents. Based on the information provided, ATSDRconcluded the site currently pose no apparent public health hazard to area residents;however, it is uncertain of what impact these limitations may impose in the future regarding long term exposure to residential well water near the site.


  1. Give consideration to removing the source areas from the Sigmon's Septic Tank ServiceFacility so as to prevent or mitigate any potential migration of chemicals into nearby private wells.

  2. Continue to routinely collect and analyze groundwater samples from both the monitoringwells and nearby privates wells surrounding the site until these source areas are removed.

  3. Periodically test well water, especially for nitrates and lead, for households with infants and small children until the source areas at the Sigmon's Septic Tank Service Facility are removed.

  4. Do not use well water from Private Wells PW2 and PW3 to prepare infant formula until it is confirmed that the nitrate levels are below 10,000 ppb. And, continue monitoring to assure that nitrate levels do not exceed 10,000 ppb in the future.

  5. Inform the residents living near the Sigmon's Septic Tank Service Facility that the Iredell County Health Department would provide an analysis of fecal and total coliform counts upon the request of any concerned well owner who feels their well has been contaminated by microbial agents.

  6. Collect the hydro-geological information required to identify the direction of groundwater flow near the site and groundwater level fluctuations with seasonal variations. Knowing the direction of groundwater flow would give better insight into which private wells are actually being impacted by the source areas located at the site.


Environmental Health Scientist:

David S. Sutton, PhD
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry


Frank C. Schnell, PhD, DABT
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry


Kathryn D. Harmsen, MPH
Office of Policy and External Affairs
Agency for Toxic Substances and Disease Registry

Reviewed by


John E. Abraham, PhD, MPH
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry

Chief, Health Consultation Section:

Susan Moore, MS
Exposure Investigations and Consultations Branch
Division of Health Assessment and Consultation
Agency for Toxic Substances and Disease Registry

Regional Representative, Region IV:

Benjamin Moore
Office of Regional Operations
Agency for Toxic Substances and Disease Registry


(*) Andelman JB. 1990. Total Exposure to Volatile Organic Compounds in PotableWater (in textbook entitled "Significance and Treatment of Volatile OrganicCompounds in Water Supplies"). Lewis Publishers. Chelsea, MI. 485-504.

*Andres P. 1984. IgA-IgG Disease in the Intestine of Brown Norway Rats IngestingMercuric Chloride. Clin. Immunol. Immunopathol. 30: 488-494.

Agency for Toxic Substances and Disease Registry. December 1989. ToxicologicalProfile for Bromodichloromethane. US DHHS, Public Health Service; Atlanta, GA.

Agency for Toxic Substances and Disease Registry. December 1990. ToxicologicalProfile for Chlorodibromomethane. US DHHS, Public Health Service; Atlanta, GA.

Agency for Toxic Substances and Disease Registry. September 1997. ToxicologicalProfile for Chloroform (Update). US DHHS, Public Health Service; Atlanta, GA.

Agency for Toxic Substances and Disease Registry. December 1998. ToxicologicalProfile for 1,4-Dichlorobenzene (Update). US DHHS, Public Health Service;Atlanta, GA.

*Agency for Toxic Substances and Disease Registry. March 1999a. ToxicologicalProfile for Mercury (Update). US DHHS, Public Health Service; Atlanta, GA.

*Agency for Toxic Substances and Disease Registry. July 1999b. ToxicologicalProfile for Lead (Update). US DHHS, Public Health Service; Atlanta, GA.

Agency for Toxic Substances and Disease Registry. August 1999c. ToxicologicalProfile for 1,2-Dichloroethane (Draft Update-Public Comment). US DHHS, PublicHealth Service; Atlanta, GA.

Agency for Toxic Substances and Disease Registry. September 2000. ToxicologicalProfile for Manganese (Update). US DHHS, Public Health Service; Atlanta, GA.

*ATSDR Electronic Mail, To: Susan Moore, Section Chief, Consultations Section,Exposure Investigations and Consultations Branch, Division of Health Assessmentand Consultation, ATSDR, Atlanta, GA, From: Benjamin Moore, Office ofRegional Operations, ATSDR, Region 4, Atlanta, GA, Date: June 27, 2001a.

Agency for Toxic Substances and Disease Registry. "Drinking Water ComparisonValue Table." US DHHS, Public Health Service; Atlanta, GA. June 30, 2001b.

*Agency for Toxic Substances and Disease Registry. ATSDR Record of Activity.Site Visit Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina. Date: July 11,2001c.

*Agency for Toxic Substances and Disease Registry. ATSDR Record of Activity.Site Visit Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina. Date: September 26,2001d.

*ATSDR Electronic Mail, To: Benjamin Moore, Office of Regional Operations,ATSDR, Region 4, Atlanta, GA, From: Giezelle Bennett, EPA, Region 4, Atlanta,GA, Date: November 19, 2001e.

*Bernaudin JF, Druet E, Druet P, and Masse R. 1981. Inhalation or Ingestion ofOrganic or Inorganic Mercurials Produces Auto-Immune Disease in Rats. Clin.Immunol. Immunopathol. 20: 129-135.

*Craun, GF, DG Greathouse and DH Gunderson. 1981. Methemoglobin levels inyoung children consuming high nitrate well water in the United States. Int. J.Epidemiol. 10(4): 309-317.

*Druet P, Druet E, Potdevin F, and Sapin C. 1978. Immune TypeGlomerulonephritis Induced by HgCl2 in the Brown Norway Rat. Ann. Immunol.129C: 777-792.

*Ellenhorn MJ and Barceloux DG. 1988. Medical Toxicology: Diagnosis andtreatment of human poisoning. Elsevier Science Publishing Company, Inc., NewYork, NY, pg. 849

Environmental Protection Agency. Guidelines for Carcinogenic Risk Assessment. Fed. Reg., 51: 33997-33998, September 24, 1986.

*EPA. 1991. U.S. Environmental Protection Agency. National Primary DrinkingWater Regulations. Code of Federal Regulations.

Environmental Protection Agency. October 1996. Drinking water regulations andhealth advisories. Office of Water. EPA 822-B-96-002.

Environmental Protection Agency. May 5, 1998a. URL: "Safe Drinking Water Fact Sheet forNitrate."

Environmental Protection Agency. December 16, 1998b. National PrimaryDrinking Water Regulations: Disinfectants and Disinfection By Products; FinalRule. Federal Register: Vol. 63, No. 241, pp 69389-69476.

*Environmental Protection Agency. 1999. "Risk Assessment Guidelines forDermal Assessment." Washington, DC.

*Environmental Protection Agency. 2001. URL: "The IEUBK."

Environmental Protection Agency, Region III Office. "Risk-Based ConcentrationTable." Philadelphia, Pennsylvania. May 8, 2001.

*Goyer, Robert A. Toxic Effects of Metals. Chap. 23 of Casarett and Doull'sTOXICOLOGY: The basic Science of Poisons. McGraw-Hill, New York, N.Y.,1996, pp 691-736.

*Integrated Risk Information System. U.S. Environmental Protection Agency,Office of Health and Environmental Assessment, Environmental Criteria andAssessment Office, Cincinnati, OH.

*Jo WK, Weisel CP, Lioy PJ. 1988. "Routes of Chloroform Exposure and BodyBurden from Showering with Chlorinated Tap Water." Risk Anal. 10:575-580.

*Kerger B and Paustenbach D. 2000. "Exposure to 1,1,1-TCE Vapors in a HomeDue to Contaminated Groundwater." Risk Anal. in press.

*Kezic S, Mahieu K, Monster AC, de Wolff FA. 1997. "Dermal Absorption ofVaporous and Liquid 2-Methoxyethanol and 2-Ethoxyethanol in Volunteers." Occup. Environ. Med. 54:38-43.

*Mattie DR, Bates GD (Jr.), Jepson GW, Fisher JW, McDougal JN. 1994. "Determination of Skin-Air Partition Coefficients for Volatile Chemicals: Experimental Method and Applications." Fundam. Appl. Toxicol. 22:51.

*North Carolina Department of Environment and Natural Resources-Division ofWaste Management-Superfund Section. Combined Preliminary Assessment/SiteInspection Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville, Iredell County, North Carolina, Reference No.06611. September 1998.

*North Carolina Department of Environment and Natural Resources-Division ofWaste Management- Superfund Section. Expanded Site Inspection Report. Sigmon's Septic Tank Service (CERCLIS No.: NCD062555792), Statesville,Iredell County, North Carolina, Reference No. 0406611. March 2000.

*National Research Council, Safe Drinking Water Committee, National AcademyPress, Washington, D.C. Drinking Water and Health. Volume 3. 1980.

*National Toxicology Program. 1993. "Toxicology and Carcinogenesis Studies ofMercuric Chloride (CAS No. 7487-94-7) in F344/N Rats and B6C3F1 Mice(gavage studies)." U.S. Department of Health and Human Services, Public HealthService, National Institutes of Health, Research Triangle Park, North Carolina. NTP TR 408, NIH Publication No. 91-3139.

Public Health Assessment Guidance Manual. US DHHS, Public Health Service;Atlanta, GA. March, 1992.

Salvato JA. 1992. Environmental Engineering and Sanitation. 4th edition. Chapter3: Water Supply.

*Simon CH, Manzke HK and GM. 1964. "Occurrence, Pathogenesis, and PossibleProphylaxis of Nitrite Induced Methemoglobinemia. Zeitschr. Kinderheilk.91:124-138. (German)

*Webster RC, Mobayen M, Maibach HI. 1987. "In Vivo and In Vitro Absorptionand Binding to Powdered Stratum Corneum as Methods to Evaluate Skin Absorptionof Environmental Chemical Contaminants from Ground and Surface Water." J.Toxicol. Eviron. Health 21:367-374.


ATSDR comparison values (CVs) are media-specific concentrations that are considered to be "safe"under default conditions of exposure. They are used as screening values in selecting site-specificchemicals for further evaluation of their public health implications. Generally, a chemical isselected for further public health evaluation because its maximum concentration in air, water, or soilat the site exceeds at least one of ATSDR's CVs. This approach is conservative by design. ATSDRmay also select detected chemical substances for further public health evaluation and discussionbecause ATSDR has no CVs or because the community has expressed special concern about thesubstance, whether it exceeds CVs or not.

It cannot be emphasized strongly enough that CVs are not thresholds of toxicity. Whileconcentrations at or below the relevant CV are generally considered to be safe, it does notautomatically follow that any environmental concentration that exceeds a CV would be expected toproduce adverse health effects. In fact, the whole purpose behind highly conservative, health-basedstandards and guidelines is to enable health professionals to recognize and resolve potential publichealth problems before they become actual health hazards. For that reason, ATSDR's CVs aretypically designed to be 1 to 3 orders of magnitude lower (i.e., 10 to 1,000 times lower) than thecorresponding no-effect levels or lowest-effect levels on which they are based. The probability thatadverse health outcomes will actually occur depends, not on environmental concentrations alone, buton several additional factors, including site-specific conditions of exposure, and individual lifestyleand genetic factors that affect the route, magnitude, and duration of actual exposures.

Listed below are the abbreviations for selected CVs and units of measure used within thisdocument. Following this list of abbreviations are more complete descriptions of the variouscomparison values used within this document, as well as a brief discussion on one of ATSDR's most conservative CVs.

CREG = Cancer Risk Evaluation Guide
EMEG = Environmental Media Evaluation Guide
LTHA = Drinking Water Lifetime Health Advisory
MCL = Maximum Contaminant Level
MCLA = Maximum Contaminant Level Action.
MRL = Minimal Risk Level
RBC = Risk-Based Concentration
RfD = Reference Dose
RMEG = Reference Dose Media Evaluation Guide

Units of Measure:
ppm = Parts Per Million [e.g., mg/L (water), mg/kg (soil)]
ppb = Parts Per Billion [e.g., µg/L (water), µg/kg (soil)]
kg = kilogram (1,000 grams)
mg = milligram (0.001 gram)
µg = microgram (0.000001 gram)
L = liter (1000 mL or 1.057 quarts of liquid, or 0.001 m3 of air)
m3 = cubic meter (a volume of air equal to 1,000 liters)

Cancer Risk Evaluation Guides (CREGs) are derived by ATSDR. They are estimated chemical concentrations theoretically expected to cause no more than one excess cancer in a million people exposed over a lifetime. CREGs are derived from EPA's cancer slope factors and therefore reflect estimates of risk based on the assumption of zero threshold and lifetime exposure. Such estimates are necessarily hypothetical for, as stated in EPA's 1986 Guidelines for Carcinogenic Risk Assessment, "the true value of the risk is unknown and may be as low as zero."

Drinking Water Equivalent Levels (DWELs) are lifetime exposure levels specific for drinkingwater (assuming that all exposure is from that medium) at which adverse, noncarcinogenic healtheffects would not be expected to occur. They are derived from EPAs RfDs by factoring in defaultingestion rates and body weights to convert the RfD dose to an equivalent concentration in drinkingwater.

Minimal Risk Levels (MRLs) are ATSDR's estimates of daily human exposure to a chemical thatare unlikely to be associated with any appreciable risk of deleterious noncancer effects over aspecified duration of exposure. MRLs are calculated using data from human and animal studies andare reported for acute (< 14 days), intermediate (15-364 days), and chronic (> 365 days) exposures. MRLs for oral exposure (i.e., ingestion) are doses and are typically expressed in mg/kg/day. Inhalation MRLs are concentrations and are typically expressed in either parts per billion (ppb) orug/m3. The latter are identical to ATSDR's EMEGs for airborne contaminants. ATSDR's MRLsare published in ATSDR Toxicological Profiles for specific chemicals.

Environmental Media Evaluation Guides (EMEGs) are media-specific concentrations that arecalculated from ATSDR's Minimal Risk Levels by factoring in default body weights and ingestionrates. Different EMEGs are calculated for adults and children, as well as for acute (<14 days),intermediate (15-364 days), and chronic (365 days) exposures.

EPA's Reference Dose (RfD) is an estimate of the daily exposure to a contaminant unlikely tocause any non-carcinogenic adverse health effects over a lifetime of chronic exposure. LikeATSDR's MRL, EPA's RfD is a dose and is typically expressed in mg/kg/day.

Reference Dose Media Evaluation Guide (RMEG) is the concentration of a contaminant in air,water, or soil that ATSDR derives from EPA's RfD for that contaminant by factoring in defaultvalues for body weight and intake rate. RMEGs are calculated for adults and children. RMEGs areanalogous to ATSDR's EMEGs.

Risk-Based Concentrations (RBCs) are media-specific values derived by the Region III Office ofthe Environmental Protection Agency from EPA's RfDs, RfCs, or cancer slope factors, by factoringin default values for body weight, exposure duration, and ingestion/inhalation rates. These valuesrepresent levels of chemicals in air, water, soil, and fish that are considered safe over a lifetime ofexposure. RBCs are calculated for adults and children. RBCs for noncarcinogens and carcinogensare analogous to ATSDR's EMEGs and CREGs, respectively.

Lifetime Health Advisories (LTHAs) are calculated from the DWEL (Drinking Water EquivalentLevel) and represent the concentration of a substance in drinking water estimated to have negligibledeleterious effects in humans over a lifetime of 70 years, assuming 2 L/day water consumption for a70-kg adult, and taking into account other sources of exposure. In the absence of chemical-specificdata, LTHAs for noncarcinogenic organic and inorganic compounds are 20% and 10%,respectively, of the corresponding DWELs. LTHAs are not derived for compounds which arepotentially carcinogenic for humans.

Maximum Contaminant Levels (MCLs) are drinking water standards promulgated by the EPA.They represent levels of substances in drinking water that EPA deems protective of public healthover a lifetime (70 years) at an adult exposure rate of 2 liters of water per day. They differ fromother protective comparison values in that they are legally-enforceable and take into account theavailability and economics of water treatment technology.

Maximum Contaminant Level Action (MCLA) are action levels for drinking water set byEPA under Superfund. When the relevant action level is exceeded, a regulatory response istriggered.

When screening individual chemical substances, ATSDR staff compare the highest singleconcentration of a chemical detected at the site with the lowest comparison value available for themost sensitive of the potentially exposed individuals (usually children). Typically the cancer riskevaluation guide (CREG) or chronic environmental media evaluation guide (cEMEG) is used. This"worst-case" approach introduces a high degree of conservatism into the analysis and often resultsin the selection of many chemical substances for further public health evaluation that will not, uponcloser scrutiny, be judged to pose any hazard to human health. However, in the interest of publichealth, it is more prudent to use an environmental screen that identifies many chemicals for furtherevaluation that may be determined later to be "harmless," as opposed to one that may overlook evena single potential hazard to public health. The reader should keep in mind the conservativeness ofthis approach when interpreting ATSDR's analysis of the potential health implications of site-specific exposures.

As ATSDR's most conservative comparison value, the CREG, deserves special mention. ATSDR'sCREG is a media-specific contaminant concentration derived from the chronic (essentially, lifetime)dose of that substance which, according to an EPA estimate, corresponds to a 1-in-1,000,000 cancerrisk level. Note, this does not mean that exposures equivalent to the CREG actually are expected tocause 1 excess cancer case in 1,000,000 people exposed over a lifetime. Nor does it mean that everyperson in that exposed population has a 1-in-1,000,000 risk (i.e., 1x10-6) of developing cancer fromthe specified exposure. Although commonly misinterpreted in precisely this way, cancer riskassessment methodology can only provide conservative estimates of population risk which do not, infact, apply to any particular individual. Even for populations, cancer risk estimates do notnecessarily constitute realistic predictions of the risk. As EPA stated in its Guidelines for carcinogenRisk Assessment, "the true value of the risk is unknown and may be as low as zero" [EPA 1986].

Unlike non-cancer comparison values which correspond to "safe" levels that include specifiedmargins of safety, ATSDR's CREGs (and the risk estimates on which they are based) correspond topurely hypothetical (and unmeasurable) 1-in-a-million cancer risk levels that include unspecifiedmargins of safety (i.e., relative to the lowest known cancer effect levels) which often range fromthousands to millions or more. In the U.S., these hypothetical risk levels are based on the zero-threshold assumption according to which any non-zero dose of a carcinogen must be associated withsome finite increment of risk, however small. Using linear models based on this assumption, it isactually possible to "quantify" undetectable/non-existent cancer risks that are (hypothetically)associated with even immeasurably small doses. EPA uses such risk estimates as regulatory toolsin, for example, the ranking of contaminated sites for cleanup. ATSDR uses them as screeningvalues. However, once ATSDR has screened a substance and selected it for further evaluation, theCREG, like all other screening values, becomes irrelevant in subsequent stages of analysis. Furtherevaluation of the public health implications of site-specific exposures must, necessarily, be based onthe best medical and toxicologic information available [PHAGM 1992].


(**)Public health Assessment Guidance Manual. U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Atlanta, GA 30333, March 1992.

**Environmental Protection Agency. Guidelines for Carcinogenic Risk Assessment. Fed. Reg., 51: 33997-33998, September 24, 1986.

Williams, Gary M., and Weisburger, John H. 1991. "Chemical Carcinogenesis".Chapter 5 in: Casarett and Doull's TOXICOLOGY: The Basic Science of Poisons.(Mary O Amdur, John Doull, and Curtis Klaassen, Editors.) Pergamon Press pp 127-200. [See section entitled "Quantitative Aspects of Carcinogenesis," pp152-155.

* Cited in text
** Cited in Appendix

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